Combustors are commonly used in industrial and commercial operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines and other turbo-machines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, multiple combustors around the middle, and a turbine at the rear. Ambient air enters the compressor as a working fluid, and the compressor progressively imparts kinetic energy to the working fluid to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel nozzles in the combustors where the compressed working fluid mixes with fuel before igniting to generate combustion gases having a high temperature and pressure. The combustion gases flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity and/or connected to the compressor to compress air.
Various factors influence the design and operation of the combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustors. However, higher combustion gas temperatures generally increase the dissociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOx). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
At particular operating conditions, some combustors may produce combustion instabilities that result from an interaction or coupling of the combustion process or flame dynamics with one or more acoustic resonant frequencies of the combustor. For example, one mechanism of combustion instabilities may occur when the acoustic pressure pulsations cause a mass flow fluctuation at a fuel port which then results in a fuel-air ratio fluctuation in the flame. When the resulting fuel/air ratio fluctuation and the acoustic pressure pulsations have a certain phase behavior (e.g., in-phase or approximately in-phase), a self-excited feedback loop results. This mechanism, and the resulting magnitude of the combustion dynamics, depends on the time between the injection of the fuel and the time when it reaches the flame zone, known in the art as “residence time” or “mixing residence time” (Tau, or T). Generally, there is an inverse relationship between residence time and frequency: that is, as the residence time increases, the frequency of the combustion instabilities decreases; and when the residence time decreases, the frequency of the combustion instabilities increases.
It has been observed that, in some instances, combustion dynamics may reduce the useful life of one or more combustor and/or downstream components. For example, the combustion dynamics may produce pressure pulses inside the fuel nozzles and/or combustion chambers that may adversely affect the high cycle fatigue life of these components, the stability of the combustion flame, the design margins for flame holding, and/or undesirable emissions.
Combustors having bundled tube fuel nozzles may experience these kinds of combustion dynamics. Previously, efforts to mitigate this problem have sought to offset the plane of fuel injection holes in some of the individual tubes within the fuel nozzle (or within the combustor head end) from the plane of the fuel injection holes in other tubes. As a result, to achieve significant difference in the residence times, the length of the tubes has been increased to accommodate multiple axially offset fuel injection planes. Such an assembly is described, for example, in U.S. Pat. No. 9,151,502.